I also like Babylon By Bus, as a good live album and an overview of his works. Beyond that, you have to spend lots more $$$. Either the aforementioned Songs of Freedom box set, which is more interesting if you want to hear the evolution of an artist, or go out and start buying each CD. Each one has its gems and they shouldn't be too expensive.
One warning for all Marley newbies: Don't expect the peace-loving, smiling, jolly fellow that Marley has been made out to be by pothead nouveau-hippies. You will find he was a serious activist and revolutionary, especially in his earlier music.